Download The Periodic Table

The Periodic Table
The Birth of the Periodic Table
In the late 1800’s, as more and more elements were discovered, the need arose to arrange them in a
pattern that would simplify their study. Mendeleev, a Russian chemist, searched for a meaningful
way to organize the elements. He decided to put them in order of increasing mass. When
Mendeleev put the elements in order by mass, he found that other properties, such as density,
seemed to repeat over and over. This type of repeating pattern is called periodic, so he called his
table the Periodic Table.
Arrangement of the Periodic Table
Mendeleev arranged the elements by increasing mass. When he did this, some of the elements’
properties didn’t match what was expected in the pattern of the periods. The periodic table we
now use takes care of that problem.
In the periodic table, each element has one more proton in its nucleus than the one before it. In
other words, element has an atomic number that is one more than the one before it. What does the
atomic number of an element tell you?
the number of protons
The information given in one box of the periodic table is shown to the
right.
The box contains four things symbol
atomic number
element name
atomic mass
What is the name of this element? Boron
What is the symbol for boron? B
What is the atomic number of boron? 5
What is the atomic mass of boron? 10.811
5
B
Boron
10.811
Atomic Mass
The nuucleus contains both protons and neutrons, each with about the same mass, 1 amu (atomic
mass unit). Since the mass of the electron is very small, compared to the protons and neutrons, you
can say the mass of the atom is simply the sum of the number of protons plus the number of
neutrons.
This number is called the mass number.
Example
An atom has 5 protons and 6 neutrons.
How many electrons does it have? 5
What is its atomic number? 5
What is its mass number? 11
What is the element? boron
If you look at the box from the periodic table for boron, you see the atomic number is five, but the
atomic mass is not 11, it is 10.811. It appears that mass number and atomic mass are not the same
thing. Which one appears in the periodic table? atomic mass
Isotopes
Imagine that you have 100 marbles. 60 are small with a mass of 1mu and 40 are large with a mass
of 2 mu. Determine the total mass of the 100 marbles.
(60 x 1mu) + (40 x 2 mu) = 140 mu
Determine the average mass of one marble.
140 mu ÷ 100 = 1.4 mu
Atoms of the same element always have the same number of protons. This identifies them as the
element that they are. But all atoms of an element don’t have to have the same number of neutrons.
For example, all boron atoms have 5 protons. However, four-fifths of them have 6 neutrons and
one-fifth of them have 5 neutrons.
What is the mass number of four-fifths of boron atoms? 11
What is the mass number of one -fifth of boron atoms? 10
Atoms of the same element with different numbers of neutrons are called isotopes of that element.
The two isotopes of boron discussed above are called boron-11 and boron-10.
How much of boron is boron-11? four-fifths (80%)
How much of boron is boron-10? one-fifth (20%)
Determine the average mass of 100 boron atoms.
(80 x 11amu) +(20 x 10amu) = 1080 amu
1080 amu ÷ 100 = 10.8 amu
What is boron’s atomic mass given in the box in the periodic table? 10.811 amu
What is the atomic mass of an element? The weighted average of all naturally occurring isotope